1. Field of the Invention
The present invention generally relates to a toner concentration control mark and in particular to a toner mark which is used to adjust toner or density in an electrophotographic printing machine.
2. Description of the Related Art
In electrophotographic printing machines such as laser printers, there may be cases where a toner concentration mark is printed on an end of a paper. In such a case, a toner concentration detector is provided to detect the toner density of the mark which is used to adjust the amount of toner supply in a developing unit in order to keep the print quality constant. Therefore, it is necessary for the toner concentration mark to precisely reflect the density of a printed image.
Such a toner concentration mark has been disclosed in Japanese Patent Examined Publication No. 62-50818. The toner concentration mark consists of a concentration control mark and an auxiliary mark which is placed before the concentration control mark. The concentration control mark is used to detect the toner concentration.
The auxiliary mark causes the well-known edge effect to be reduced. However, the detection output of the concentration control mark is not always stable because of deteriorated photosensitive drum, worn transfer unit, or other problems. Therefore, there may be developed cases where the toner concentration control is not properly performed.